a) Field of the Invention
The invention concerns additives which may be used as lamination lubricating agents or are part of lamination lubricating agents. The invention also concerns compositions including these additives and which may be used in the lamination of a sheet such as lithium in order to obtain thin films, which may be used as such in the production of polymer electrolyte electrochemical cells. In addition, the invention concerns the use of the additives per se or compositions containing same to provide, by lamination, films of alkali metals or alloys thereof which may be used as anodes in electrochemical cells preferably with polymer electrolytes. The invention also concerns a process of lamination utilizing these additives or compositions containing same as lamination lubricating agents.
b) Description of Prior Art
The production of thin films of lithium having thicknesses lower that 75 micrometers and in the form of wide bands, for example 5 centimeters or more and in lengths of many tens of meters, by means of rapid and reliable processes, faces important technical difficulties which are attributable to the extreme physical and chemical properties of this metal: chemical reactivity, malleability, rapid self-welding by simple contact and strong adhesion on most solid materials, for example the usual metals.
This difficulty is confirmed by the difficulty of obtaining from suppliers of specialty metals and chemical products, thin lithium films 40 micrometers (.mu.m) thick and less, of sufficient surface and length, having an adequate surface finish and chemical property to be used in lithium cells.
Presently, cold extrusion is used for the continuous production of sheets 75 .mu.m and more. These thicknesses are generally adapted to the production of lithium cells utilizing liquid electrolytes. For lower thicknesses, the films obtained by extrusion are thereafter laminated between rollers made of hard materials. These processes have been described and are commercially used for the production of limited quantities of sheets of 30-75 microns. Reference will particularly be made to U.S. Pat. No. 3,721,113, inventor Hovsepian and dated Mar. 20, 1973. Many successive passes, according to the present state of the art, are required to give films 40-30 .mu.m.
Other alternative processes have been described to give ultra-thin sheets, which are used for example in the production of polymer electrolyte cells in the form of thin films. This is the case for example of a lamination process between steel rollers which are protected by films of hard plastic which are non reactive towards lithium, such as described in U.S. Pat. No. 3,721,113, or of processes based on the coating of molten lithium on a metallic of plastic support, described in U.S. Pat. No. 4,824,746, inventors Andre Belanger, et al, dated Apr. 25, 1989.
The difficulty in achieving the lamination of lithium to thicknesses which vary between 40 and 5 microns for the production of polymer electrolyte cells is mainly due to the reactivity and the adhesion of the laminated metal with the materials with which it is in contact: lamination rollers, protection plastic films, lamination additives, as well as to the bad mechanical properties of thin sheets. For example, a film of lithium 20 .mu.m thick and 10 cm wide breaks under a drawing tension higher than 579.13 KPa which does not permit to pull on the film which exits from the laminating machine or to release it from the lamination rollers if lithium adheres somewhat thereto.
An approach which is normally used for the extensive lamination or calandering of hard metals, such as iron and nickel, is based on the use of liquid lamination additives consisting of organic solvents which may contain greases or lubricating agents. Examples include fatty acids or derivatives thereof such as for example lauric or stearic acids and alcohols, for example the compounds known under the trade marks EPAL 1012 of Ethyl Corporation U.S.A., which are mixtures of primary linear C.sub.10 -C.sub.12 alcohols.
For lithium and particularly for lithium intended for electrochemical cells, the use of such additives involves two major difficulties:
1) the chemical reactivity of lithium which is in contact with solvents or lubricating agents including reactive organic functions, such as organic acids and alcohols. These functions react at the surface of lithium during and after lamination and create passivation films at the surface of the metal. This is harmful for a good operation of electrochemical cells especially when the latter are intended to be rechargeable; PA1 2) the difficulty of removing the lubricating agents or greases which are in contact with lithium after lamination. This is the case, for example, when lubricating agents which mostly consist of hydrocarbon chains are selected, because they are nearly not reactive with lithium. These compounds constitute electrical insulating materials which are harmful to the good operation of lithium electrodes made with these sheets. Such lubricating agents are not very soluble in polymer electrolytes and should therefore be removed from the surface of lithium by washing after lamination. In addition to the fact that the washing of the surface of lithium is a delicate and costly operation, it will be noted that this operation inevitably contributes to contaminate the surface of lithium, in spite of all the care which may be used to control the quality of the surface of the metal. The latter reacts indeed irreversibly with all the impurities, including water, which are present in the washing solvents, or resulting from accidental contaminations. PA1 L designates a hydrocarbon radical, such as alkyl, alkylene, linear or cyclic or aryl-alkyl, saturated or non saturated, preferably containing more than 8 carbon atoms used as a lubricating segment which is compatible with lithium; PA1 S designates an oligomer segment including heteroatoms such as O or N, and capable of solvating salts, for example salts of lithium and ensuring an electrolytic conductivity; PA1 Y designates a chemical bond or a chemical group which is at least divalent joining the chains or chain segments L and S. PA1 Polymer Electrolytes review-1, J. R. MacCallum & C. A. Vincent eds. Elsevier Applied Science London (1987); PA1 Polymer Electrolyte, review 2, J. R. MacCallum & C. A. Vincent eds. Elsevier Applied Science London (1989); PA1 Solid Polymer Electrolytes, F. M. Gray VCH Publisher New-York, Weinheim (1991); as well as PA1 Surface Active Ethylene Oxide Adducts, by V. Schoenfeldt-Permagon Press, (1966).
It can be shown that the lithium obtained after a process of lamination with an additive followed by a subsequent washing is generally more contaminated at the surface than a lithium which is laminated without additive. This phenomenon may be observed with optical means, including a simple visual inspection or by a control of the impedance of the electrochemical batteries produced with polymer electrolytes. On the other hand, lamination without solvent and without lubricating agent means low production speeds and a tendency of the fresh lithium to stick to the rollers or the protection films of the rollers; moreover, many consecutive laminations are therefore required to reach thicknesses of the sheet lower than 40 micrometers.